Argonauta argo, also known as the greater argonaut, is a species of pelagic octopus belonging to the genus Argonauta. In some parts of Guangdong, China, this species is locally known as “白海马巢” While the name literally translates as "white sea-horse's nest", the term "sea-horse" in regional Sinitic languages actually refers to sand crabs (such as ghost crabs) rather than the animal commonly known as a seahorse, and bears no relation to horses. In Taizhou, local elderly speakers refer to its shell as "sea-horse shell", while the soft body is called "octopus".
A. argo was the first argonaut species to be described and is consequently the type species of the genus. The type specimen of A. argo was collected in the Mediterranean Sea and is deposited at the Linnean Society of London.
Description
A. argo is the largest species in the genus and also produces the largest eggcase. Live animals have a characteristic blue sheen on the first arm pair and around the eyes.
A damaged beak of a female A. argo (ML = 40.0 mm; caught at ), measuring 4.3 mm in hood length and 7.8 mm in crest length, is mentioned in A Handbook for the Identification of Cephalopod Beaks.
Distribution and habitat
A. argo is cosmopolitan, occurring in tropical and subtropical waters worldwide.
thumb|240px|left|Female with damaged eggcase and eggs
Males of this species reach sexual maturity at a mantle length (ML) of 8 mm. Females mature at about double the size of Argonauta bottgeri and Argonauta hians. A small A. argo residing in an 88 mm long eggcase was estimated to be carrying 48,800 embryos. Females grow to 100 mm ML, while males do not exceed 20 mm ML. This behaviour has been known for a long time, although little was understood about the relationship prior to the work of Heeger et al. in 1992. In "Predation on jellyfish by the cephalopod Argonauta argo", Heeger et al. describe their observations of a female A. argo found atop a host jellyfish. The argonaut was seen holding on to the aboral (exumbrellar) surface of the jellyfish using its lateral and ventral arms. The authors found that about half of the animal's aboral surface was damaged and large pieces of mesoglea were missing, presumably removed by the argonaut. Additionally, two holes, apparently bite marks, were found in the center of this area with channels leading from these holes into the gastral cavity of the jellyfish. The argonaut presumably used these channels to suck food particles from the gastral cavity. Heeger et al. suggested that "the association provided shelter or camouflage for the argonaut".), which means "Shell of Aoi" (not Malva but Asarum caulescens). Japanese have compared two eggcases of argonauts lapped each other to a leaf of Asarum caulescens.